1. Technical Field
The present invention relates to valves, and in particular, to valves for use with turbine engines.
2. Description of the Related Art
Turbine engines have a set of rotating turbine blades that compress air into the combustion area where fuel is injected and ignited. Combustion of the fuel turns a downstream set of blades from which energy is extracted and which can also be used to drive the compressor blades.
The combustion area of the turbine can be divided by and contained in combustion cans. There are anywhere from six to eighteen such cans in the large turbine systems used to generate power. Each can has a burner to ignite the fuel as well as injectors for injecting the fuel into the can for combustion. Typically, some of the injectors are designated as primary injectors and one or more are secondary injectors. The primary injectors are used during the start-up sequence and at periods of lower fuel consumption, but otherwise both the primary and secondary injectors are used during normal operation of the turbine.
Many turbine systems are designed for dual fuel operation, such that either a liquid fuel, such as diesel fuel, or a gaseous fuel, such as natural gas, is consumed when the turbine is operating. The liquid fuel, for example, could be delivered to the combustion cans via the primary and secondary injectors mentioned, and one or more additional gas injectors could be provided at each can for delivery the gaseous fuel during operation in gas mode.
When the engine is to be shut down or when the engine is operated in gas mode, the fuel pump supplying liquid fuel is turned off, a fuel line valve is closed and the fuel line and the injectors are purged with hot gases from the turbine compressors. Because the injectors are adjacent the burners and thus extremely hot, the relatively cool, yet still hot purging air cools the injectors and reduces “coking” at the injector orifices, which can occur when volatile components of the fuel are driven off by heat leaving only a tarry deposit. Purging is necessary to prevent the injectors from being damaged as well as to ensure that the orifices and valves are clear of coke deposits, which could inhibit proper delivery of the fuel when the turbine is returned to fuel mode.
In order to permit purging of the liquid fuel line, a purging air line must join with the fuel line. It is important that the fueling and purging operations be isolated so that fuel does not go down the purge air line and hot gases do not travel up the fuel line to the fuel supply. Check valves are often installed in each of these lines to prevent this. However, common spring-loaded checks may be unreliable, sticking in the open position or allowing some backwash into either of the fuel or purge lines. Alternatively, the fuel cut-off valve may be operated by a pneumatic actuator in line with the check valve of the fuel line, however, this is also subject to coking and may introduce substantial pressure drops in the fuel flow. The fuel cut-off valve also introduces a potential failure point to the turbine where, if pressure is lost to the pneumatic actuator, the turbine will cease operating.
A three-way purge valve is disclosed in U.S. Pat. No. 6,050,081, assigned to the assignee of the present invention and hereby incorporated by reference as though fully set forth herein, which provides a significantly more reliable mechanism for air purging and fuel control, both preventing backwash and being more resistant to coking. As disclosed, a spool valve having an enlarged middle section was used to shuttle between positions alternatively blocking the combustion can from either the purge air line or the fuel line. The spool is biased to close off the fuel line and is urged to open the fuel line by a pilot air actuated piston. Thus, when fuel is to be closed off from the engine, the spool valve will return to its initial position thereby allowing the burner nozzles and the downstream side of the spool to be purged to reduce or eliminate coking in these areas.
To avoid the need to use dedicated purge valves for each injector, a separate distributor can be mounted to each combustion can to act as a manifold to which the several fuel lines connect before the fuel is routed to the individual injectors. This additional component and additional lines add significant cost, assembly and size to the system. And, these parts create additional areas for coking to occur, particularly given that the distributor is typically mounted directly to the combustion can which realizes extreme temperatures during combustion.
A distributor three-way purge valve is disclosed U.S. Pat. No. 6,931,831, assigned to the assignee of the present invention and hereby incorporated by reference as though fully set forth herein, which overcomes the aforementioned problem by combining the functionality of the three-way purge valve with that of a distributor in a single unit.
While these valves provided a significant improvement to turbine fuel systems, it was still necessary to separately control metering to the injectors since these valves had no metering capabilities and the fuel was delivered into the combustion cans through open metering orifices. Thus, separate flow dividers and shear valves were required. Further, the valves themselves became additional components which add to the complexity and thus decrease the reliability of the system.